1. Field of the Invention
The present invention relates to a loudspeaker diaphragm and a method for manufacturing such a diaphragm. More particularly, the present invention relates to a loudspeaker diaphragm which is light weight and possesses an excellent balance between rigidity and internal loss, as well as a simple and inexpensive method for manufacturing such a diaphragm.
2. Description of the Related Art
Generally, properties which are required for a loudspeaker diaphragm include a high Young's modulus (a high elastic modulus or rigidity) and an appropriate internal loss (tan δ). A known means for improving Young's modulus, is a diaphragm employing FRP (Fiber Reinforced Plastic), i.e., a composite of a carbon fiber and an epoxy resin, which is a typical example. A known means for improving an internal loss is typically a diaphragm employing a synthetic resin such as polypropylene.
The above-mentioned diaphragms respectively have a problem. Specifically, the FRP diaphragm has a high Young's modulus. However, since an epoxy resin (a matrix resin of FRP) has a very small internal loss, an internal loss of a diaphragm is small as a whole. As a result, such a diaphragm tends to cause a resonance and therefore has frequency characteristics in which a so-called peak dip appears very much. Accordingly, it is quite difficult to prevent production of sound which is inherent in a diaphragm material. Regarding the synthetic resin diaphragm, in many cases, it has satisfactory frequency characteristics due to its large internal loss. However, the synthetic resin diaphragm has insufficient rigidity and heat resistance.
As means that improves a balance between a rigidity (Young's modulus) and an internal loss, a diaphragm employing a polyethylene naphthalate film is proposed in, for example, JP 01-067099 A and JP 06-181598 A.
Furthermore, since a request for reducing the weight of a diaphragm has recently become intensive, various attempts have been made. For example, a light weight diaphragm, which has an unfoamed structure on the surface portion and a foamed structure at the inner portion and which is obtained by using a thermoplastic resin to which a foaming agent is added and by adjusting clamping force on a mold cavity and a mold clearance at the time of performing an injection molding, is proposed in JP 3135482 B. Alternatively, as an attempt that simultaneously satisfies mechanical strength and reduction in weight, a foamed resin product, which has two cell structures respectively having a different foam density, is proposed in JP 11-080408 A. This foamed resin product is obtained by impregnating a resin with carbon dioxide gas having concentration gradient in a supercritical state and by heating the impregnated resin to be foamed.
However, techniques described in the above-mentioned publications respectively have a problem as follows. A technique described in JP 01-067099 A and JP 06-181598 A is applicable only to a loudspeaker having a small diameter (i.e., a so-called micro speaker). More specifically, according to the technique described in these publications, it is possible to obtain a diaphragm having sufficient rigidity and internal loss for being used for a micro speaker. However, since an internal loss of such a diaphragm is extremely insufficient for being used for a loudspeaker having a large diameter, it is impossible for the technique to obtain a practically acceptable diaphragm used for a loudspeaker having a large diameter.
According to a technique described in JP 3135482 B, it is extremely difficult to adjust the time at which a foaming is performed and the time at which the clamping force and mold clearance is varied. As a result, it is difficult to stably obtain a diaphragm having a satisfactory balance between mechanical strength and weight. According to a technique described in JP 11-080408 A, since a resin molded product (e.g., a sheet) is impregnated with gas, it requires an excessive amount of time to be sufficiently impregnated with the gas. For example, in the case where a resin having high crystallinity is used for improving mechanical strength, it may take 100 hours or more for gas impregnation. Therefore, this technique is not at all commercially practicable.
As mentioned above, a loudspeaker diaphragm which is light weight and possesses excellent balance between rigidity and internal loss in a variety of uses (i.e., regardless of the diameter of a resultant loudspeaker), coupled with a simple and inexpensive method for manufacturing such a diaphragm has been in great demand.